Virtually all modern firearm ammunition uses a metallic case to hold the elements of the cartridge together before firing. The cartridge case also serves to seal the chamber of the firearm to prevent leakage of the high-pressure gases that result from the burning of the powder charge. Due to the high pressure inside the cartridge case, the case is stretched to fill the full dimensions of the chamber in which it is contained, and the case retains this enlarged size after firing.
Despite this stretching, cartridge cases can be reused. Resizing and other reloading operations are done using dies placed in manually operated presses. Adjustments are made by threading the body of the die up and down in the press. A threaded lock ring is then used to secure and retain the position of the die in the press once a proper adjustment setting is achieved. This adjustment of the die—either toward or away from a ram in the press—is used in most reloading operations, including sizing, forming, and seating of the bullet onto the cartridge case.
Proper die adjustment is critical to the functioning and safety of the reloaded ammunition. Many individuals prefer to adjust their dies to an accuracy of 0.001″ (one thousandth of an inch). However, the current state of the art does not allow for such fine adjustments by a measured amount, leaving the individual to adjust the die settings by trial and error. Without a controlled way to thread the dies into and out of the press in precise increments, multiple attempts at the correct adjustment are required until the desired adjustment is reached.